The present invention relates to a position or proximity sensing system which has its most important application as an anti-collision control system for overhead cranes.
Many different types of position detecting and control systems have been developed for the purpose of preventing one object from striking another. Included among some of the present developments in this field are devices which are intended to prevent aircraft on a collision course from continuing on that course by automatically redirecting them and establishing new conditions of flight to prevent collision of the craft. Land vehicles are also involved in this field of interest, both tracked and untracked. Some of the simpler approaches to proximity sensing anti-collision systems involve the use of cushioned bumpers or other fixed barriers to prevent vehicles from destructively bumping into walls or other objects. Some complicated systems involve the use of radar techniques, including doppler radar, or the use of photo-optics or other electronic or optical sensing means to determine the position and rate of travel of one object with respect to another. Sophisticated control devices have the characteristic drawbacks of being difficult to use and to maintain in use. Also, they are expensive to manufacture and in this sense the use of such devices usually is restricted where economics is a factor in installation and use.
As previously indicated the present invention is particularly adapted for use as an anti-collision system in conjunction with cranes, either electrical overhead or other crane systems. In one crane anti-collision system heretofore developed, each crane carried a transmitter unit including a tone generator and an amplitude level monitoring circuit, the frequency generated by the tone generators being in the low audio range. The system also included a pair or wires strung parallel to and along the crane runway. The outputs of the tone generators in the cranes were connected to the wires strung along the runway and a bridge circuit formed, in part, by the wires produced a signal whose amplitude varied with the position of the cranes. This system is unsatisfactory because the cost of running wires parallel to the path of travel of the cranes and the use of pick-up shoes and associated apparatus would involve a substantial installation and maintenance expense, and raise signal pick-up problems which would make the control system marginal from a reliability and economical standpoint.
The provision of position detecting and control devices for cranes must also take into consideration the operating characteristics of the cranes themselves. That is, cranes frequently are involved in carrying heavy loads pendulously supported below the superstructure of the crane. Accordingly, the crane must be slowly maneuvered to a stop in use or the load may swing violently and may cause derailment of the crane, dropping of the load, or other damage which might be less desirable than if the collision were permitted to occur.
It is also important that the anti-collision system be so designed that it interferes to a minimum degree with the ability of the operator (who for overhead crane installations is generally on the ground and supplied with radio equipment to control the crane remotely) to maneuver the crane, though slowly, adjacent to other cranes or other objects with which it can collide. Thus, while it is important for an anti-collision system to stop a crane approaching another crane or a stationary object at a given speed before it reaches a point where it cannot readily be stopped in time to avoid collision, such systems have heretofore not been designed so that the operator can operate the crane slowly at distances which could cause serious collisions when the crane is moved at a normal speed. However, to make a crane anti-collision system responsive to a particualr speed at which the crane is moving imposes requirements on the anti-collision system which can render it too expensive and complex to make it a readily marketable product.
In addition to the factors just described, manifestly the crane system must be a reliable one which will not be adversely effected by noise and interfering signals. For example, while there have been attempts at designing anti-collision systems for cranes and the like utilizing radio signals between transmitting and receiving antennas mounted on the cranes and stationary portions of the crane trackway, such systems have proved unreliable because of the false operation thereof from interfering signals generated by impulse noise from adjacent electric motors and the like or radio transmitters of ham radio operators or on passing vehicles. Also, some previously proposed radio signal operated anti-collision systems would be rendered inoperable by variations in signal strength caused by signal reinforcement or cancellation due to reflections from adjacent objects. Thus, a radio anti-collision system which measures the spacing between the transmitting and receiving antennas as a function of signal strength at the receiving antenna are particularly subject to false operation by impulse noise interference and signal reinforcement and cancellation due to signal relections from surrounding objects.
Accordingly, it is a general object of the present invention to provide a relatively inexpensive vehicle proximity sensing and control system which performs certain control functions at pre-determined distances between the vehicle and other objects and wherein ambient electrical noise and radio signals will not interfere with proper functioning of the system.
Another object of the present invention is the provision of an improved proximity sensing and control system as above described that is economical to install, easy to maintain, and readily adjustable to different monitoring distances.
A further object of the present invention is to provide a proximity sensing and control system as described which is useful, for example, to prevent collision between a crane and other cranes or stationary objects at the end of a crane trackway without interfering with the ability of the crane operator to move the crane, though slowly, into close proximity to other cranes or the end of a crane trackway. A related object of the invention is to provide a proximity sensing and control system useful for collision prevention or the like, wherein different control operations are to be performed for widely differing monitoring distances. A still further related object of the invention is to provide a radio controlled crane anti-collision system or the like which is not adversely effected by signal attenuation or reinforcement problems caused by re-reflected radio waves.